1) JKLM is a rectangle --- if a parallelogram contains one right angle, then all other angles are right angles and its a rectangle.
2) JM = LK , LM = JK ---- Opposite sides of a parallelogram are congruents.
3) JM = JK = LM = LK ---- Transitive property
4) JKLM is a square ----- Defn of a square (4 right angles and all sides are congruent)